1. Field of the Invention
The present invention relates to an abrasion-resistant welded steel pipe suitablly used in portions or parts of industrial machines, apparatus or devices related to mining, or transportation of slurry material such as ore slurry and coal slurry, the portions or parts receiving severe abrasion.
2. Description of the Related Arts
In general, an abrasion-resistant steel can be improved in its resistant property by increasing its surface hardness. For this reason, most of the prior art abraision-resistant steels have been produced by quenching steels containing a great quantity of alloy elements in addition to carbon. Such abrasion-resistant steels, however, have disadvantages in being poor in workability due to their high hardenability and in weldability due to their high carbon equivalent.
To overcome these disadvantages, several prior arts are disclosed in respect of abrasion-resistant steel plates. One is a method for producing abrasion-resistant steel plates, by hot-rolling steels having considerably low carbon equivalent and then, quenching the hot-rolled steel plates. This is disclosed in such publications as Japanese examined patent publication No. 56-14127, Japanese unexamined patent publication No. 57-89426 and Japanese patent unexamined Publication No. 61-76615. Those produced abrasion-resistant steel plates have a high Vickers hardness of 350 or more, due to the quenching treatment. This high hardness gives the high abrasion-resistant property to the hot-rolled plates. It is, however, well known that when the hardness is increased exclusively by such heat treatment, a zone heat-affected by welding is remarkably softened due to the heat produced at welding. When steel pipes are produced from steel plates by welding, the steel plates are formed into a cylindrical shape and then, a seam portion of both edges of the cylindrically formed plates are required to be welded. Therefore, when steel plates are used for production of welded steel pipes, it is expected that the heat-affected zone of the welded steel pipes are significantly softened and that the abrasion-resistant property of the heat-affected zone is deteriorated. In addition, these disclosed steel plates are expected to be poor in the workability and the weldability and they are difficult to use to produce abrasion-resistant welded steel pipes.
Further, an abrasion-resistant laminated steel plate comprising a steel sheet with high hardness and a steel sheet with low hardness is disclosed in Japanese unexamined patent publication No. 3-227233. The abrasion-resistant property of the laminated steel plate is procured by the steel sheet with high hardness located on the surface side and the weldability of the laminated steel plate is procured by the steel sheet with low hardness located on the lower side. The laminated steel plate, however, is disadvantageous in that the steel sheet with high hardness forming a hardened layer of the laminated steel plate is hard to use for a long period in an environment where the hardened layer is easy to be worn out since the hardened layer is thin in thickness and in that producing a steel pipe by welding the laminated steel plate is extremely difficult.
In Japanese unexamined patent publication No. 63-290616, a method for producing an abrasion-resistant steel pipe from a laminated steel plate, a steel sheet forming the hardened layer being made from an austenitic stainless steel or a martensitic stainless steel. This abrasion-resistant steel plate has also a problem in that the steel plate is costly because it contains 12 wt. % or more Cr, beside the already mentioned disadvantages peculiar to the laminated steel plate.